Several methods have been known for cutting out a blank of desired shape from a material sheet. A widely used shearing method which cuts blanks out of freely supported material sheet, and a fine-blanking method which cuts blanks out of material sheet which is firmly fixed by means of a projecting sheet holder, are typical examples.
When continuously cutting out a plurality of blanks from a single material sheet or plate, the former known method needs webs as carriers and bridges that support the material. The metal left over from these webs is scrap, thus greatly lowering the blank-to-material yield (to between 60 and 70 percent). Besides, the bending moment arising at the moment of blanking bends the periphery of the cut-out blank, causing deformation thereof. Specifically, the sheared surface is not right-angled but is tapered with respect to the sheet surface due to the springback of the blank, which thus heavily damages the dimensional accuracy of the blank. Further, the cut edge assumes a complex shape because of the effect of shear droop and deformation, fracture, burrs and so forth (see FIG. 7).
The latter known method provides a blank contour which suffers little blanking deformation since it controls the occurrence of shear droop, fracture and burrs. This naturally results in improved dimensional accuracy. However, wide carriers and bridges must be left to insure contact with a projecting sheet holder. This increases the quantity of scrap and seriously lowers the product-to-material yield (to as low as between 30 and 50 percent; see FIG. 8).
In addition, the material must have high ductility and toughness as well as a uniform, closely packed structure. In being blanked, such material exerts greater pressure against the working face of the tool, causing heavy wearing and shortening of the tool life.
Therefore, the latter method is generally inferior to the former method in respect to productivity and economics.
Now this invention has obviated these shortcomings associated with the aforesaid conventional methods. The method of blanking according to this invention cutting a V-shaped endless groove of desired contour on the top surface, or on both the top and bottom surfaces, of a material sheet so that the external side of the groove is at right angles to the top surface or both top and bottom surfaces of the material. A desired blank is then obtained by precision-shearing along the V-shaped groove by such methods as blanking, punching, lancing and parting.
The following describes embodiments of this invention by reference to the accompanying drawings.